Solar cell modules comprise several tens of solar cells which are electrically arranged in tandem or parallel and are provided with a protective package to make them into a unit. Generally, solar cell modules have a structure in which the surface facing to sunlight is covered with a transparent glass or film, solar cells are encapsulated with a filler comprising a light transmissive ethylene-vinyl acetate copolymer resin (EVA) as amain component, and the back side thereof is protected with a laminated sheet (back sheet) formed with a weather resistant plastic material and a metal thin membrane.
The solar cell modules, as they are used outdoors and always exposed to sunlight, i.e., they are exposed to harsh conditions, are required to have sufficient durability in terms of the quality of materials and structures. Among others, back sheets are required, in order to prevent detachment of a filler, discoloration or corrosion of wirings, to have weather resistance (resistance to ultraviolet, moisture resistance, heat resistance, salt damage resistance etc.), water vapor barrier properties, electrical insulation properties, mechanical strength, chemical resistance, compatibility to be adhered and unified with the filler and the like. Particularly, it is required that they can prevent penetration of water, oxygen gas etc., have minimum degradation of the performance for a prolonged period of time, have high durability and are safe and low in cost.
Recently, thin-film solar cells are attracting attention in order to reduce cost and increase efficiency of solar cells, and among others, thin-film silicon solar cells such as of amorphous silicon and crystalline silicon are in practical use. Moreover, flexible solar cells such as CdTe (cadmium telluride), CIS/CIGS (copper indium selenide/copper indium gallium selenide) are also under development for practical application. Flexible solar cells are lightweight and have preferable processability, so that they have wide applications such as for terrestrial type, roof top type, building integrated type as well as for home electronics and for in-vehicle system, for which the market is expected to grow dramatically. Protective packages are required to have weather resistance and flexibility, and technical challenges are to improve their properties.
It has been studied to use resin films such as acrylic resins, polyolefin resins, polyvinyl chloride resins, fluororesins and polyester resins as back sheets. However, only the single film does not meet the requirements in various properties such as weather resistance, heat resistance, moisture proof properties and the like. For example, PET films which are low in cost and versatile and have excellent mechanical strength have a water vapor transmission rate of 14.4 g/m2·day (comparative data from Example 1 of the present application), which is not tolerable for long-term use under outdoor environment. There is a need for obtaining a film which can be used as a protective film or back sheet of devices that are used under harsh conditions, e.g., solar cell modules, under severer conditions than conventional materials. Flexible solar cell modules are further required to have mechanical strength such as flexibility, bending properties, etc.
Most of the conventional back sheets having excellent flexibility, water vapor barrier properties and gas barrier properties have been prepared by a method in which a resin film substrate is provided with a barrier layer(s) on one or both sides thereof. Barrier layer of aluminum oxide, silicon oxide, silicon nitride or the like is formed by various methods such as CVD or PVD method. However, water vapor barrier properties and gas barrier properties of the thus formed barrier layers are not always perfect. Because heat resistance depends on the heatproof temperature of a resin, it has been required to use inorganic sheets or metal sheets as gas barrier materials having higher heatproof resistance.
Inorganic sheets are obtained by processing natural or synthetic minerals such as mica or vermiculite into the form of a sheet and have high heat resistance. Although they are used at least for gas sealing members such as gland packings, they are not densely molded and thus cannot completely block the paths where minute molecules of gas flow, thereby having rather low gas barrier properties. Although metal sheets have excellent gas barrier properties, their applications are limited because they have drawbacks in weather resistance, electrical insulation properties, chemical resistance etc.
Various organic polymer materials are used and incorporated to molding materials as a gas barrier material, as well as to inorganic materials as a dispersant, thickener or binder. For example, it has been known that a membrane which is formed with a composition comprising 100 parts by weight of a mixture of (A) a carboxyl-containing resin having hydrogen bonding tendency and having two or more carboxyl groups in a molecule such as polyacrylic acid and (B) a hydroxyl-containing resin having hydrogen bonding tendency and having two or more hydroxyl groups in a molecular chain such as starch in the weight ratio of A/B=80/20 to 60/40 and 1 to 100 parts by weight of an inorganic layered compound such as clay mineral and which has a thickness of 0.1 to 50 μm has gas barrier properties after heat treatment and electron beam treatment (see Patent Document 1). However, in this case, a main component is the additive resin and heat resistance depends on the properties of the polymer organic material.
Laminated films having excellent moisture proof properties and gas barrier properties so as to be applicable for food packages and the like can be obtained by laminating, between two polyolefin resin layers, a layer formed with a resin composition comprising an inorganic layered compound and a resin (see Patent Document 2). Heat resistance of this type of laminated membranes depends on the organic material having the lowest heat resistance among the materials contained, which is in this case polyolefin, and therefore this type of material cannot generally have high heat resistance.
For the purpose of improving heat resistance and gas barrier properties of organic polymers or resins, various clays such as smectite, mica, talc, vermiculite, etc. are added as a filler to resins and their effects have been validated. However, smectite, which has high water dispersibility, is hydrophilic and therefore have low affinity to hydrophobic resins, making it difficult to disperse and incorporate smectite itself at a high amount in resins. Therefore, in order to incorporate it into hydrophobic resins, clays are modified and used in order to control hydrophilicity/hydrophobicity thereof (see Non-patent Document 1).
One of the methods for producing modified clay is ion exchange by a quaternary ammonium cation or quaternary phosphonium cation. Hydrophilicity/hydrophobicity can be controlled by controlling the type and proportion of these organic cations introduced (see Non-patent Document 2). Another method is silylation. Because clay crystals contain hydroxyl groups at the terminals, the hydroxyl groups react with a silylating agent added to make the terminals hydrophobic. In this case again, hydrophilicity/hydrophobicity can be controlled by controlling the type and proportion of the silylating agent introduced. These two modification methods may be used in combination.
Various methods for forming membranes of clay have been reported including a method for preparing an inorganic layered compound thin membrane based on the Langmuir-Blodgett method (see, for example, Non-patent Document 3) and a method for preparing a functional inorganic layered compound thin membrane and the like. Various examples are known such as a method for producing a clay thin membrane by forming a membrane from an aqueous dispersion of a hydrotalcite intercalation compound and drying the same (see Patent Document 3), a method for producing a clay mineral thin membrane having the oriented and fixed binding structure of the clay mineral by utilizing the reaction between the clay mineral and phosphoric acid or a phosphate group and facilitating the reaction by applying heat (see Patent Document 4), an aqueous composition for membrane treatment comprising a smectite type clay mineral and a complex compound of bi- or multivalent metal (see Patent Document 5).
However, in any of those conventional methods, an inorganic layered compound-containing free standing oriented membrane has not been obtained which has mechanical strength sufficient to be used as a free standing membrane and is imparted with gas barrier properties by highly orienting the layers of clay particles. The present inventors invented and suggested a basic method for preparing a free standing clay membrane by simple means as described below (see, for example, Patent Document 6).    (1) Clay particles are dispersed in a liquid dispersion medium to prepare a homogeneous clay dispersion.    (2) The dispersion is poured in a tray or the like and left to precipitate clay particles, and the liquid dispersion medium is separated by a solid-liquid separating means to form a clay membrane.    (3) The clay membrane is optionally dried under the temperature conditions of 110 to 300° C. to obtain a free standing clay membrane.
The present inventors have proposed, in the fields of packages, sealants, electrical insulating materials and the like, a material formed of a modified clay membrane which has gas barrier properties, water vapor barrier properties as well as water resistance and has mechanical strength sufficient to be used as a free standing membrane by adding to a main component of a modified clay a small amount of an additive (Patent Document 7). However, this modified clay membrane has not been in fact in practical use in the fields of solar cells and electronic devices such as displays, fuel cells and the like because further gas barrier properties and water vapor properties, flexibility and mechanical strength, high heat resistance, water resistance and weather resistance are required.
Patent Document 1: Japanese Patent Application Laid-open No. H10-231434
Patent Document 2: Japanese Patent Application Laid-open No. H7-251489
Patent Document 3: Japanese Patent Application Laid-open No. H6-95290
Patent Document 4: Japanese Patent Application Laid-open No. H5-254824
Patent Document 5: Japanese Patent Application Laid-open No. 2002-30255
Patent Document 6: Japanese Patent Application Laid-open No. 2005-110550
Patent Document 7: Japanese Patent Application Laid-open No. 2007-277078
Non-patent Document 1: Masanobu Onigata, SMECTITE, vol. 8, No. 2, p. 8-13 (1998)
Non-patent Document 2: Masanobu Onigata, SMECTITE, vol. 13, No. 1, p. 2-15 (2003)
Non-patent Document 3: Yasushi Umemura, Journal of the Clay Science Society of Japan, vol. 42, No. 4, p. 218-222 (2003)
Under these circumstances with the forgoing in view, the present inventors have carried out extensive studies aiming to develop a new water vapor barrier film utilizing a clay membrane which satisfies the requirements in the improved gas barrier properties and water barrier properties, flexibility and mechanical strength, high heat resistance, water resistance and weather resistance, as described above, and can be put into practical use in the fields of solar cells and electronic devices such as displays, fuel cells and the like and as a result, have found a preferable modified clay by improving the previously developed modified clay and a suitable additive to be added thereto; preferable mixing ratio of the modified clay and the additive; preferable solid-liquid ratio of a dispersion; preferable dispersion method and the like. The present inventors have also found that a laminated product obtained by applying the modified clay paste on a resin film can significantly improve water vapor barrier properties and gas barrier properties of the resin film and increase adhesiveness between the resin film and the clay membrane, and thus completed the present invention.